I think someone already did a good rundown of this, but I’ll see if I can take a stab at it.
Cassidy spent his entire life secondary, unwanted, un-cared-for, not in control. The last of these became especially direct, so to speak, when Woody abused him, and whatever feelings he had at that point of being wanted and cared for were horribly tainted by the abuse he suffered. Woodman said that he just wanted to be there for those kids, that they were so badly neglected. He convinced himself of that, and while Cassidy knew better, it was still quite likely the most affection he was getting at the time. Like dying of thirst and drinking dirty dishwater, no matter how bad it was and how terrible it tasted, it was all he had.
Couple that with a perpetually absent mother – I got the feeling from the one scene with her that she was interested in nothing so much as herself, that she had no particular desire to be tied down by mothering for even a little while – and a father and brother who probably teased him unmercifully starting pre-puberty that he was gay – and you have the beginnings of a severely troubled individual. Whether he really WAS gay or straight doesn’t really matter – his sexuality, however oriented, had been twisted by that earlier abuse. Sex was a truly dirty and filthy thing to him, and that’s troublesome in a teenager overcome with hormones day and night… especially with a brother like Dick.
School? Friends? He seemed to have a few friends, but for the most part the school didn’t see him as having an identity of his own. He was Dick’s brother. Beaver. A feminizing name, possibly started by his brother, and when the rest of the world started calling him that, they all became Dick to him.
He could have gone two ways – he could have withdrawn completely into himself, into self-destructive activity, but that would have been obvious. The Dicks might not have noticed, but his classmates and his teachers and the school counselor might have, and it would make it so much more likely for all this to come out into the open. And at heart I’m pretty sure Cassidy was a sweet, decent kid… just with a really terrible dark side that started coming out just before Season 1, when he finds Veronica drugged and half-consicous.
I think this really was a turning point for him, where he chose to ape his Incredibly Heterosexual father and brother. He did what either one of them would have done, what they urged him to do. He was hoping it would, as they thought, Make Him A Man. But afterward he was still just Beaver. No matter what, he was always going to be Beaver.
The bus crash was all about power. He had no empathy for the kids on the bus – most, if not all, of those kids had made fun of him. What did he care what happened to them? They were all extensions of his brother, even the ones who’d been on his team. He got to drive behind the bus knowing all the while he had their lives in his hands. The same with the bombs in Woody’s house and car and plane – I have no doubt he put them all there quite some time before (keeping a charge on the phones might, I admit, have been difficult), walking around every day knowing that with one quick call he could kill the man. He didn’t have to kill him on any particular day… just knowing he COULD gave him power over his abuser.
I think he still had something of a conscience. Just not while he had the power. When he lost the gun on the roof, his personality shifted dramatically. He was a scared kid again, afraid and in pain and anguish.
But I think he really did care about Mac. She was sweet and clever and smart, she treated him like a human being… she was the only drop of normality in his life. I think he didn’t want to “dirty” her with sex – having a girlfriend without having to worry about all the messiness of sex would have been very appealing. The whole ‘sex’ thing was probably a bit of a personal test for him: if he could have completely normal ordinary everyday non-drug-assisted sex with a girl he was quite fond of, he could pretend he was completely normal and unaffected. The control he most desired would be his – control over himself.
I think stealing her clothes was, in a rather sick way, his way of both punishing her and protecting her. She wanted to sleep with him, so she was bad, and he knew the humiliation of being naked and alone quite well. But on the other side, there was no chance of her trying to follow him or alerting everyone else that he’d gone missing. He could have as much time on the roof with Veronica as he wanted without fear of being discovered.
This is all just… rambling, I guess, but as much as people have said they didn’t like Beaver’s final performance, I wanted to say I felt I understood it. The posturing and the mustache-twirling nastiness was not the entirety of Cassidy… it was just his darkest side mixed with some 9 mm bravado.